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Balázs Lécz added a comment - 01/Mar/12 03:20 PM After executing this test, look at the datanucleus.log file and check the datastore access pattern.
In the case of MySQL, 2 individual SELECTs are generated, instead of a signle one using an IN-list.

Chris Colman added a comment - 07/Mar/12 09:09 AM - edited In general, when performing a query over all classes on a particular branch of a class hierarchy I seem to recall, historically at least, seeing SQL where clauses like:
where (classid=discrim1 or classid=discrim2 or classid=discrim3)
Using an IN statement for the query might have a moderate impact on query size but it would likely have a bigger impact on readability.
For example the following is more concise and readable and has less repetition:
where classid in (discrim1,discrim2,discrim3)
This is especially so for very large and deep class hierarchies (like we have in some places in our code base).